Abstract: Hydrogen blending in natural gas serves as a transitional technology for the large-scale utilization of hydrogen and the low-carbon transition of urban gas systems. By blending a certain proportion of hydrogen into existing natural gas pipeline networks, this approach promotes the integration of green hydrogen and the supply of low-carbon gas, while also improving the utilization efficiency of existing gas infrastructure. In recent years, Europe, North America, and China have successively launched natural gas hydrogen blending demonstration projects. The blending ratio has gradually expanded from early low-ratio trials to verification of medium-to-high ratios, while application scenarios have extended from urban gas sectors such as residential heating and domestic hot water to industrial combustion, gas turbine power generation, transportation fuels, energy storage and peak shaving, island microgrids, and integrated regional energy systems. Existing research and engineering practices indicate that low-ratio hydrogen blending demonstrates good engineering feasibility under specific conditions regarding low-pressure distribution networks, gas source composition, and end-use appliances. However, as the blending ratio increases, issues such as hydrogen embrittlement of pipeline materials, seal leakage, hydrogen permeation, gas-to-gas interchangeability, combustion stability, NOx emissions, flow metering, calorific value correction, online monitoring, and safety regulation will become more prominent. Based on domestic and international natural gas hydrogen blending demonstration projects and relevant research findings, this paper reviews the development background, current status of demonstrations, key technical challenges, evolution of application scenarios, progress in policies and standards, and future trends of natural gas hydrogen blending. The study concludes that natural gas hydrogen blending in China is currently at a critical stage of transition from single-site engineering demonstrations to standardization, large-scale implementation, and commercialization. Future efforts should focus on strengthening research into scenario-specific blending ratio limits, long-term operational data, compatibility with existing pipeline networks, certification of end-use equipment, energy metering systems, and carbon emission accounting methods to support the safe, economical, and orderly promotion of natural gas hydrogen blending in urban gas systems.